At the present time all industrial plants for the production of a molten iron catalyst for the synthesis of ammonia are based on three process schemes which contemplate the manufacture of a crushed and granulated catalyst.
As the starting product for the catalyst use may be made of iron, naturally-occurring or synthetic magnetite produced by oxidation of pure iron of the "Armco" type with additives of K.sub.2 O, CaO, Al.sub.2 O.sub.3, SiO.sub.2 and the like.
Since the catalyst quality depends to a considerable extent on purity of the starting materials, it is a common practice nowadays to make use of synthetic magnetite, wherein the content of harmful impurities (S, P, Cl, Cu, Ni and the like) is much lower than in naturally-occurring magnetite.
The first process scheme is based on the production of the catalyst from synthetic magnetite.
The production line consists in the following steps performed in succession: oxidation of the catalyst iron to magnetite by gaseous oxygen without the addition of promotors; cooling of the melt; crushing, in jaw and roll crushers, of the synthetic magnetite; grinding thereof in a ball mill; blending of synthetic magnetite with activators in a special mixer; melting of the catalyst charge in electric-resistance furnaces; cooling of the molten catalyst; crushing thereof in jaw and roll crushers and screening of the final ground catalyst to commercial fractions.
The most critical operations are the production of synthetic magnetite by way of oxidation of the catalyst iron with gaseous oxygen and melting of the prepared charge in electric-resistance furnaces.
All these numerous operations are performed discontinuously in the prior art processes (cf. U.S. Pat. No. 1,554,008; V. P. Kamzolkin, N. V. Kul'chitsky "Synthesis of Ammonia", Moscow, 1940; V. P. Lipinskaya in Col. "Scientific Foundations for the Selection and Manufacture of Catalysts", Published by U.S.S.R. Academy of Science Publishers, Siberian Division, 1964, Novosibirsk, p. 109).
The second production scheme for the manufacture of a molten catalyst for the synthesis of ammonia contemplates the use of a two-stage melting and involves six successive operations: melting of the catalyst iron in a crucible of an induction furnace; oxidation of the iron melt and addition of promoters in an oxidation crucible lined with magnesite bricks; cooling; crushing of the catalyst in jaw and roll crushers and screening of the final crushed catalyst to commercial fractions (U.S.S.R. Inventor's Certificate No. 206554, 1966).
This prior art production line operates in the following manner.
The catalyst iron is manually charged into an induction furnace crucible in the amount of 150 kg; therein, under the effect of high-frequency current it is melted and the resulting liquid metal is cast into an oxidizing crucible.
The oxidizing crucible comprises a spherical bath lined with refractory magnesite bricks. In the crucible conical portion a manhole is provided for maintenance and visual inspection of the process. Gaseous oxygen is fed into the crucible from the top.
The bottom portion of the oxidizing crucible is charged with the catalyst fines obtained from the preceding melting cycles along with the required amount of promotors.
The iron melted in the induction furnace is cast, by gravity, through a tapping hole into the prepared mould of the oxidizing crucible. Oxygen for oxidation is fed as soon as the liquid metal starts to fill the crucible.
An intensive oxidation of the molten iron and dissolution of promoters in the melt occur for 15-20 minutes.
After each oxidation operation a layer of the catalyst fines is again charged onto the crucible bottom along with promoters intermixed with the fines and this periodic process is repeated. The stages of cooling, crushing and screening of the catalyst are effected using conventional methods. Therefore, the above-described production schemes comprise a combination of certain discontinuous operations and are not suitable for automation, wherefore they feature a low efficiency.
Furthermore, the crushed catalyst produced in these processes when practically used for the synthesis of ammonia results in a non-uniform distribution of a gas stream, local overheatings in the column for the synthesis of ammonia and evolution of dust during handling and operation stages, thus impairing quality of the resulting ammonia and lowering productivity of the synthesis columns.
The third production scheme for the manufacture of the catalyst involves five successive operations: oxidizing melting of iron with promoters; granulation of the catalyst melt by means of an activating liquid; washing of the catalyst to remove the activating liquid; drying and screening of the catalyst.
The first operation in the production scheme is the oxidizing melting of iron with promoters which is conducted in a crucible with water cooling in a stream of gaseous oxygen (cf. U.S.S.R. Inventor's Certificate No. 38135, Cl. B 01 J 23/74, 1934; FRG Pat. No. 957475, 1957).
Oxidation of iron in this process is effected portionwise, while the required promoters are alternatively introduced into the melt after oxidation of each of the following portions. The resulting melt of the catalyst mass is poured from the crucible for 3-6 seconds. This rapid casting of the melt is explained by the fact that the reaction of the heat evolution upon oxidation of iron is completed and the catalyst mass may get cooled in the crucible which is considered as an emergency situation in the present stage.
The following stage, i.e: granulation of the molten catalyst mass obtained in the preceding stage is effected by feeding same into the activation liquid such as liquid potassium glass with the addition of potassium carbonate preliminary prepared in a separate unit and charged into a granulator. The molten catalyst mass is fed through spinnerets or a die plate with orifices intended for distribution of the melt stream to a number of thin jets and granulation.
Inside the granulator a rotary hollow drum is located which is half-immersed in the liquid; a stirrer for agitation of the activation liquid is also provided.
The molten catalyst mass from the crucible is poured within 3-6 seconds through a die plate onto the granulator drum (cf. U.S.S.R. Inventor's Certificate No. 177856, Cl. B01 J 11/12).
The molten catalyst mass is fed through the die plate is effected into a strongly turbulized stream of the activation liquid (cf. U.S.S.R. Inventor's Certificate No. 476018, Cl. B01 J 23/74, 1975).
The basic disadvantage of granulation of the molten catalyst into the activation liquid resides in a considerable difficulty of the production of a catalyst having a stable chemical composition during the manufacture due to dissolution of the catalyst promoters in the activation liquid and the inverse process of invasion of the activation liquid components into the catalyst. This phenomenon necessitates additional stages of washing the catalyst granules to remove the activation liquid and drying of the granules.
Due to the periodic character of the process of preparation of the molten catalyst mass, the granulation process in this production scheme is also batch-wise.
Furthermore, granulation of the molten catalyst mass at a temperature of at least 1,600.degree. C. (which is the melting point of the catalyst mass) in an aqueous liquid may be accopanied, in the case of non-compliance with the process requirements, by a thermal decomposition of water with the formation of a fulminating mixture, thus always bearing the risk of explosion.
Consequently, all currently employed production schemes for the manufacture of a catalyst for the synthesis of ammonia feature a complicated multi-stage process technology which does not enable automation thereof and hinders a total mechanization and performance of discontinuous processes. Furthermore, granulation of the catalyst in a liquid which is effected in the third process scheme does not ensure a total safety of the process. All the industrial schemes for the production of the catalyst for the synthesis of ammonia are designed for its manufacture in the oxidized state. In the use of such a catalyst in the industry of nitrogen an additional operation is required, namely: reduction thereof to .alpha.-Fe which should be carried out either in the synthesis column or in a special plant; this also entails certain additional capital expenses.